Feat: world performance

[sandexzx]

Summary

Replace lowMemoryMode boolean with worldPerformance enum setting.

What

• Add worldPerformance option with three levels:
  low-energy (1 worker), normal (3 workers, dedicated change),
  maximum (4–8 workers based on hardwareConcurrency)
• Remove lowMemoryMode toggle and hidden numWorkers setting
• Migrate old lowMemoryMode: true → worldPerformance: 'low-energy'
  in both migrateOptions and migrateOptionsLocalStorage
• Update watchOptions to compute mesherWorkers and
  dedicatedChangeWorker from the selected performance level
• Disable dedicatedChangeWorker when WASM mesher is active
• Show "World Performance" in Settings → Render → Experimental
  with requiresRestart flag

Why

The old binary toggle (1 vs 4 workers) had no middle ground.
The new 3-level setting gives users a balanced "Normal" mode
with dedicated change worker for responsive block updates,
plus battery-saving and maximum-performance extremes.

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